Protective external coatings are sometimes applied to printed circuit boards that are intended for use in field environments. Throughout this disclosure, the application of such protective external coatings to printed circuit boards will be referred to as "overcoating." One such protective overcoating technique is to cover the finished board with a conformal coating. Another technique is to overmold the finished board with a rubber-like elastomeric material. The latter technique is used not only to provide protection against moisture and other environmental conditions to which the board may be exposed during use, but is also sometimes used to create elastomeric structures that are helpful in securing the finished board to a frame or enclosure.
Unfortunately, the materials used to create such protective external coatings can interfere with the operation of certain areas of a printed circuit board. For example, the printed wiring traces on many printed circuit boards are designed to form spark gaps in certain areas so that high electrostatic charges may be safely shunted to ground through the gap. In order for such spark gaps to function properly, they must be free of the conformal coat or elastomer material used to seal the board. In addition, the spark gap must be free of other contaminants such as aerosols and other hydrocarbons commonly associated with the application of the conformal coat or elastomeric materials.
One prior technique that attempted to produce an elastomer-overmolded printed circuit board having a contaminant-free spark gap is illustrated in FIGS. 1-3. Two copper printed circuit traces 112, 114 were formed on an epoxy-fiberglass composite substrate 100 separated by a narrow space 118. A layer of soldermask 116 was applied to the printed circuit board, but was left open in aperture 117 containing spark gap area 118. A 0.050 inch diameter pin (hereinafter "shutoff pin") 120 was pressed down onto the assembly as shown at arrow 126. A support pin 124 was pressed against the assembly from the opposite side, as shown at arrow 128, to counteract the force of shutoff pin 120. Once shutoff pin 120 and support pin 124 were in place, elastomer was applied to the entire assembly. Typically, the elastomer Ha was applied under high pressure, between 100 and 300 p.s.i. The intention was that the presence of shutoff pin 120 would prevent intrusion of elastomer into aperture 117 and spark gap area 118 during molding. Unfortunately, the profile of soldermask layer 116 over printed circuit traces 112, 114 caused a thin (approximately 0.001 inch) wide gap 130, 132 around the perimeter 122 of shutoff pin 120. The high-pressure elastomer easily infiltrated under shutoff pin 120 into gap 130, 132, causing contamination of aperture 117 and spark gap area 118.
Although theoretically it might be possible to reduce the elastomer intrusion problem by more perfectly aligning the perimeter of shutoff pin 120 with aperture 117, such a solution would require unfeasibly fight mechanical tolerances. Current manufacturing technology is only capable of aligning printed circuit features to within +/-0.005 to 0.015 inches of other mechanical features such as edges and alignment holes. This level of precision would be inadequate to align shutoff pin 120 closely enough with aperture 117 to prevent infiltration of overcoating material into the area of interest.
It is therefore an object of the present invention to provide a method of overcoating a printed circuit board while preserving certain contaminant-free areas.
It is a further object to provide such a method that also avoids the above-described tolerance problems, thus resulting in easier and less expensive manufacture.
It is a further object of the present invention to provide a method of constructing a contaminant-free spark gap on an overcoated printed circuit board.